We reported the outbreak of a Pseudomonas aeruginosa strain producing VIM-2 type MBLs in burn wards, which happened after the transfer of a patient with P. aeruginosa infection.

The outbreak of carbapenem-resistant P. aeruginosa occurred in four different burn wards of the Jilin Factory Hospital in China during the period of July to September 2006. The first carbapenem-resistant P. aeruginosa strain was isolated from the burn culture of a 6 year-old child at admission. The patient was transferred from a local hospital for burn injury on the leg. He had received broad-spectrum antibiotics, including ampicillin-sulbactam and piperacillin-tazobactam at the local hospital. His antibiotic was switched to impenem for pneumonia coverage, admitted to the general burn ward. After one week, blood and burn cultures yielded P. aeruginosa isolates susceptible only to aztreonam and he died from sepsis and multiple-organ failure soon thereafter. Subsequently, the carbapenemresistant P. aeruginosa strain was recovered in seven other patients hospitalized in the general burn ward.

A total of eight clinical isolates of carbapenem-resistant P. aeruginosa were consecutively recovered from patients hospitalized at the burn wards. Routine antibiograms were performed by the disk diffusion method according to the guidelines of the Clinical and Laboratory Standards Institutes.1 The resistance patterns of the eight strains are illustrated in Table 1. Among the antimicrobial agents tested, four isolates were only susceptible to ciprofloxacin and aztreonam. Metallo-β-lactamases production was evaluated using the imipenem-EDTA disk method.2 All isolates were positive in the EDTA disc synergy test indication the presence of an MBL.

PCR analyses were performed with whole genomic DNA from all case isolates. Primers used for analysis of blaVIM /blaIMP genes were Vim F/R, Imp F/R respectively. PCR primers were designed and PCR conditions were performed as previously described.3 Association of integrons with MBL genes was confirmed by PCR using combination of blaand integron-specific primers as described previously.4

PCR products of blaVIM/blaIMP were electrophoresed on a 1.5% agarose gel. Nucleotide sequencing was performed directly on cloned fragments using an ABI Prism 377 DNA sequencer. Sequence homology was performed using the BLAST program available at the website of the National Center of Biotechnology Information.

PCR using the VIM forward and VIM reverse yielded an internal fragment of approximately 650 bp suggesting the presence of a blaVIM gene, and sequencing of its PCR product was consistent with blaVIM-2, while no other metallo-β-lactamases was detected. Furthermore, by combining the primer attI1 with VIM-R and attI1 with VIM-F, colinearity of blaVIM-2 genes with class 1 integrons was detected in all isolates.5

Genomic DNA, prepared as described previously and digested with SpeI was subjected to PFGE with the CHEF DRIII. Comparison of the PFGE profiles with those strains isolated in burn wards indicated that the epidemic strains were identical (data not shown).6

The emergence of the blaVIM-2 gene at the hospital was rapid, and the patients had strong epidemiological links. In addition, the blaVIM-2 - carrying isolates, which were recovered from the eight patients during the single hospitalization, were the same strain, suggesting a nosocomial transmission. The spreading of carbapenem resistance, mediated by VIM-2 MBL in P. aeruginosa isolates in burn wards, is largely due to clonal expansion of a VIM-2-carrying strain throughout the burn wards, which was carried on a mobile gene cassette inserted into a class 1 integron located on the bacterial chromosome.

Contaminated hands of health personnel and colonized or infected patients are sources of infection. Therefore, constant glove changing between patients and proper hand sanitization should be enforced.7

Follow-up testing did not reveal the presence of the epidemic strains in burn wards as well as in the hospital settings studies for the following half year.

ACKNOWLEDGEMENTS

This project was supported by grants from the National Science & Technology Program during the Eleventh Five-Year Plan Period in China (2004BA720A09-02).